If the next -and final- puzzle piece has an 'E', then we have some ideas of what words we can form with all 15 letters.
Could you share some of these ideas?
Well, I'll share one I came up with (which means the idea will likely be useless

):
The letters from the 14 puzzle pieces obtained so far are:
D, S, Y, R, F, R, O, G, T, P, L, A, N, K
If the next and final letter is an 'E', then an anagram you can form is:
GRONSFELD KRYPTA
Freddy has said "
I really enjoy Gronsfeld Ciphers", but so far we've only had one of that sort of ciphers to solve.
Plus, the guy who invented the cipher was German, and 'krypta' is a word in German - it translates in English as 'crypt', which is a term used not only for chambers and sarcophagi, but for computer science and passwords as well. See:
https://en.wikipedia.org/wiki/Crypt_(C)
https://en.wikipedia.org/wiki/Crypt_(Unix)
('KRYPTA' or an anagram of it could also be a word encrypted through the Gronsfeld cipher, which we'd need to crack.)
Of course, 'GRONSFELD' could be a coincidence; or, maybe we don't even get an 'E' in the first place!
My two cents, anyway.
(If we get an 'I', an anagram is 'DYSTOPIAN RFRGLK'.)